RF12 Universal ISM Band FSK Transceiver

RX FIFO BUFFERED DATA READ
RF12
In this operating mode, incoming data are clocked into a 16 bit FIFO buffer. The receiver starts to fill
up the FIFO when the Valid Data Indicator (VDI) bit and the synchron pattern recognition circuit indicates
potentially real incoming data. This prevents the FIFO from being filled with noise and overloading the
external microcontroller.
Polling Mode:
The nFFS signal selects the buffer directly and its content can be clocked out through pin SDO by
SCK. Set the FIFO IT level to 1. In this case, as long as FFIT indicates received bits in the FIFO, the
controller may continue to take the bits away. When FFIT goes low, no more bits need to be taken. An
SPI read command is also available.
Interrupt Controlled Mode:
The user can define the FIFO level (the number of received bits) which will generate the nFFIT when
exceeded. The status bits report the changed FIFO status in this case.
FIFO Read Example with FFIT Polling
During FIFO access f SCK cannot be higher than f ref /4, where f ref is the crystal oscillator frequency.
CRYSTAL SELECTION GUIDELINES
The crystal oscillator of the RF12 requires a 10 MHz parallel mode crystal. The circuit contains an
integrated load capacitor in order to minimize the external component count. The internal load
capacitance value is programmable from 8.5 pF to 16 pF in 0.5 pF steps. With appropriate PCB layout,
the total load capacitance value can be 10 pF to 20 pF so a variety of crystal types can be used.
When the total load capacitance is not more than 20 pF and a worst case 7 pF shunt capacitance (C 0 )
value is expected for the crystal, the oscillator is able to start up with any crystal having less than 300
ohms ESR (equivalent series loss resistance). However, lower C 0 and ESR values guarantee faster
oscillator startup.
The crystal frequency is used as the reference of the PLL, which generates the local oscillator
frequency (f LO ). Therefore f LO is directly proportional to the crystal frequency. The accuracy requirements
for production tolerance, temperature drift and aging can thus be determined from the maximum
Tel: +86-755-86096587 Fax: +86-755-86096602 E-mail: sales@hoperf.com http://www.hoperf.com